Virtual Network -Public IP

We are migrating an application to azure , this application talks to the on premise WCF /DB- We are considering to use Virtual network to have the connectivity.


The on premise WCF is also talking to many other on premise application.

In this case , What would be the impact on other application?
When we configure virtual networks with a particular Ip range(subnet) - what would be the impact on all other applications talking to this WCF ?


Also , since the on premise server has to be public facing (IP) - what are the security impact ? what are the mitigations that can be taken care ?